Chest pain in patients with angiographically normal coronary arteries may be caused by a limited coronary flow response to stress because of abnormal function of the coronary microcirculation (microvascular angina). Because dyspnea is common in these patients and seems disproportionate to the severity of myocardial ischemia, we studied airflow in the basal state and after methacholine inhalation to determine whether bronchial smooth muscle is affected in this syndrome. Nineteen of 35 patients (53%) with microvascular angina had abnormal resting or methacholine-provoked airflow responses, a significantly greater prevalence of abnormal airflow studies compared to 9 patients with heart disease (O%, p>0.025) and 24 normal volunteers of similar age and gender distribution (13%, p>0.025). Furthermore, the product of the methacholine dose inhaled and the magnitude of decline in forced expiratory volume in one second (FEVI) from baseline was significantly lower in patients with microvascular angina than in normal volunteers. We conclude that airway hyporesponsiveness is frequently demonstrable in patients with microvascular angina; these findings are consistent with the hypothesis that this syndrome may represent a more generalized abnormality of vascular and nonvascular smooth muscle function.